Type aliasing is a feature in some programming languages that allows creating a reference to a type using another name. It does not create a new type hence does not increase type safety. It can be used to shorten a long name. Languages allowing type aliasing include: C++, C# Crystal, D, Dart, Elixir, Elm, F#, Go, Hack, Haskell, Julia, Kotlin, Nim, OCaml, Python, Rust, Scala, Swift and TypeScript.
C++ features type aliasing with the using keyword.
using Distance = int;
C# version 12 and higher supports type aliasing with the using keyword. Earlier versions restrict its use to file-local scope or specific import contexts..
using Distance = int;
Crystal features type aliasing using the alias keyword.
alias Distance = Int32;
D features type aliasing using the alias keyword.
alias Distance = int;
Dart features type aliasing using the typedef keyword.
typedef Distance = int;
Elixir features type aliasing using @type.
@type Distance :: integer
Elm features type aliasing using type alias.
type alias Distance = Int
F# features type aliasing using the type keyword.
type Distance = int
Go features type aliasing using the type keyword and =.
type Distance = int
Hack features type aliasing using the newtype keyword. Functionally, this creates a new, distinct type that is incompatible with its underlying type (int). This is stricter than a simple alias, which is generally transparent and interchangeable with the original type.
newtype Distance = int;
Haskell features type aliasing using the type keyword.
type Distance = Int;
Julia features type aliasing. The use of const is best practice (though not strictly required for aliasing). It prevents the alias from being rebound to a different type later in the program, ensuring the alias is stable.
const Distance = Int
Kotlin features type aliasing using the typealias
typealias Distance = Int
Nim features type aliasing.
type
Distance* = int
OCaml features type aliasing.
type distance = int
Python features type aliasing.
Vector = list[float]
Type aliases may be marked with TypeAlias to make it explicit that the statement is a type alias declaration, not a normal variable assignment. The use of : TypeAlias (from PEP 613) is not required for the alias to function, but it explicitly tells static type checkers (like Mypy) that the assignment is a type declaration, not a runtime variable assignment.
from typing import TypeAlias
Vector: TypeAlias = list[float]
Rust features type aliasing using the type keyword.
type Point = (u8, u8);
Scala can create type aliases using opaque types.
object Logarithms:
opaque type Logarithm = Double
Swift features type aliasing using the typealias keyword.
typealias Distance = Int;
TypeScript features type aliasing using the type
type Distance = number;
Zig features type aliasing by assigning a data type to a constant.
const distance = u32;